Abstract
Microstructure and mechanical properties of novel Ni-20Co-12Cr superalloys, with and without Nb addition, were systematically studied during long-term thermal exposure. With increased exposure time, the average diameter of the γ′ precipitates increased in both alloys in succession; this is more obviously observed in alloy containing 1 wt% Nb (1Nb). It is suggested that Nb increased the γ′ coarsening rate by accelerating the diffusion of Al and Nb in γ matrix. In addition, the γ′ phase fraction is increased by about 4% in 1Nb compared to the alloy without Nb (0Nb). The morphology of the γ′ phase changed from near-spherical to cuboidal shape during exposure in both alloys. Due to the increased γ/γ′ lattice misfit by Nb addition, 1Nb alloy showed an earlier tendency of shape change. Vickers hardness results revealed that the overall hardness decreased with the exposure time because the size increment of the γ′ precipitate weakened the precipitates strengthening and Orowan strengthening.
Highlights
Powder metallurgy (PM) nickel-based superalloys demonstrate high fatigue resistance, hot-corrosion resistance, high tensile and stress-rupture properties, as well as high oxidation resistance up to 900 ◦ C, and have been widely used as turbine disc materials in aeroengines and power generation turbines [1,2,3]
The argon atomization (AA) powders of the 0Nb and 1 wt% Nb (1Nb) alloys were sieved to a final screen size of 150 mesh and filled into mild steel containers and hot isostatically pressed (HIP) under 1180 ◦ C/150 MPa for 4 h
After standard heat treatment (SHT), specimens were exposed in air at 800 ◦ C up to Change of microstructure owing to thermal exposure was characterized by field emission scanning electron microscopy (SEM; FEI Quanta 650 FEG, Brno, Czech Republic)
Summary
Powder metallurgy (PM) nickel-based superalloys demonstrate high fatigue resistance, hot-corrosion resistance, high tensile and stress-rupture properties, as well as high oxidation resistance up to 900 ◦ C, and have been widely used as turbine disc materials in aeroengines and power generation turbines [1,2,3]. The typical microstructure of nickel-based superalloys includes disordered γ matrix and ordered γ0 -Ni3 (Al, Ti, Nb) precipitates. Al, Ti, Ta, and Nb are known as the γ0 forming elements. W, Cr, Mo are known as solid solution strengthening elements.
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